Support case when user e-mail is not unique

[apragacz]

or at least inform library user about that, for instance show at least a Django system check error.

This is actually a default setting in django.contrib.auth.models.User - the email field is NOT unique.

The current state of Django-REST-Registration wouldn't lead to any security issues AFAIK, but it may lead to various errors like:

• Not all users receiving (or even all users not receiving) an e-mail when reset password is made.
• Users unable to login via e-mail if it is added to the 'LOGIN_FIELDS' setting key.